Singing Sand Dunes: The Mystery of Desert Music

If you've never heard a sand dune rumble, listen up. Marco Polo in the
13th Century said the singing sands -- which he ascribed to evil
desert spirits -- "at times fill the air with the sounds of all kinds of musical
instruments, and also of drums and the clash of arms."

Yes, certain sand dunes will occasionally let out a loud, low-pitch rumble
that lasts up to 15 minutes and can be heard up to 6 miles (10 kilometers) away.
Some dunes are known to do it regularly, even daily. But why?

To try and uncover the underlying nature of these mysterious sounds, Bruno
Andreotti from the University of Paris-7 took equipment out to the Atlantic
Sahara in Morocco, one of only 35 known places where the mysterious natural
music can be heard.

"Singing dunes constitute one of the most puzzling and impressive natural
phenomenon I have ever encountered," Andreotti said.

Setting the stage

Andreotti and his team studied one of the large crescent-shaped dunes, or
barchans, which spontaneously sings all year long - sometimes two or three times
an afternoon, if windy enough.

Wind forces sand to accumulate at the top of the dune until the angle of the
slope reaches a tipping point of about 35 degrees. The eventual avalanche of
sand produces the bellowing noise. The sand must be sufficiently dry for the
singing to occur. For smaller barchans, the sand must also be hot and the wind
still.

"A small dune sings only the few days in which there is no wind and no clouds
so that the Sun can dry efficiently the slip face," Andreotti told
LiveScience.

Although sand avalanches were known to be the cause of the singing, the exact
mechanism was still unclear. Not wanting to wait for a spontaneous episode,
Andreotti and his team induced avalanches in the field by sliding down the
dunes.

Nature's boom box

By measuring vibrations in the sand and air, Andreotti was able to detect
surface waves on the sand that emanated from the avalanche at a relatively slow
speed of about 130 feet per second (40 meters per second). In this way, the face
of the dune acts like a huge loudspeaker - with the waves on the surface
producing the sound in the air.

Andreotti explained these sand waves as resulting from collisions that occur
between grains at about 100 times per second, as measured in the lab. In a kind
of feedback loop, the waves synchronize the collisions, so they are all on
basically the same beat.

This model explains the low pitch - between 95 and 105 Hertz - of the sand
song, which, according to Andreotti, resembles a drum or a low-flying propeller
aircraft.

The feedback mechanism, as outlined by the researchers in the Dec. 1 issue of
Physical Review Letters, also correctly predicts the maximum loudness of
the singing to be 105 decibels, at which point the sand grains vibrate off the
surface. This level of sound is comparable to a snow blower or a Walkman at full
volume.

The mystery is not completely solved. Recent research has centered on a
seemingly magical musical property of the singing grains. It is not known, for
instance, why the sliding of glass beads is silent, while some rougher sand
grains belt out a tune.

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Author Bio

Michael Schirber

Michael Schirber began writing for LiveScience in 2004 when both he and the site were just getting started. He's covered a wide range of topics for LiveScience from the origin of life to the physics of Nascar driving, and he authored a long series of articles about environmental technology. Over the years, he has also written for Science, Physics World, andNew Scientist. More details on his website.